The integration of urea and melamine production has long been known. Melamine is thereby produced from urea, according the following reaction:6(NH2)2CO→C3H6N6+6NH3+3CO2 
Interestingly, ammonia and carbon dioxide result from this process in precisely in the stoichiometric ratio from which these substances form urea.
The latter is generally presented in the form of two consecutive reaction steps. In the first step, ammonium carbamate being formed according to the exothermic reaction:2NH3+CO2→H2N—CO—ONH4 
Thereafter the formed ammonium carbamate is dehydrated in the second step to give urea according to the endothermic equilibrium reaction:H2N—CO—ONH4H2N—CO—NH2+H2O
In the art, urea plants are generally filled with starting materials on the basis of an excess of ammonia (i.e. above the 2:1 stoichiometric ratio). These reactants are subjected to a pressure between 12 and 40 MPa and a temperature between 150° C. and 250° C. in a urea synthesis zone.
In the integrated production of urea and melamine, urea produced in a urea synthesis zone is sent to a melamine synthesis zone. The carbon dioxide and ammonia off-gas resulting from the melamine production is, in turn, recirculated as a starting material for use in the urea synthesis zone.
It will be understood that, irrespective of the excess of ammonia introduced, the reactants are drawn from the synthesis zone in not more than a 2:1 ratio, and the off-gas from the melamine synthesis thus reintroduces the urea-forming reactants in the same ratio. Thus, the ammonia to carbon dioxide ratio in the entire urea and melamine synthesis and recirculation loop remains the same.
The integrated production can take place in an integrated plant, having urea and melamine synthesis zones. More typically, however, the melamine and urea synthesis zones are comprised in separate plants for the production of urea and melamine, which plants are coupled using the appropriate flow lines so as to realize the aforementioned integration.
A typical urea plant is a urea stripping plant. Therein, the decomposition of the ammonium carbamate that has not been converted into urea and the expulsion of the usual ammonia excess largely takes place at a pressure that is essentially almost equal to or lower than the pressure in the synthesis reactor. This decomposition and expulsion take place in one or more stripper(s) installed downstream of the reactor, possibly with the aid of a stripping gas such as, for example, carbon dioxide or ammonia, and with the addition of heat. The gas stream leaving a stripper contains ammonia and carbon dioxide which are condensed in a high-pressure condenser, operating at essentially equal pressure to the pressure in the stripper and then returned to the urea synthesis zone.
In a urea stripping plant the synthesis zone is operated at a temperature of 160-240° C. and preferably at a temperature of 170-220° C. The pressure in the synthesis reactor is 12-21 MPa, preferably 12.5-20 MPa. The ammonia to carbon dioxide molar ratio (N/C ratio) in the urea synthesis zone of a stripping plant lies usually in between 2.2 and 5 and preferably between 2.5 and 4.5 mol/mol. The synthesis zone can be carried out in a single reactor or in a plurality of reactors arranged in parallel or series.
After the stripping treatment, the pressure of the stripped urea solution is reduced in a urea recovery section (or recirculation section, as from this section carbamate is recirculated). In a recovery section the non-converted ammonia and carbon dioxide in the urea solution is separated from the urea and water solution. A recovery section comprises usually a heater, one or more liquid/gas separation sections and one or more condensation sections. The urea solution entering a recovery section is heated to vaporize the volatile components ammonia and carbon dioxide from that solution. The heating agent used in the heater is usually steam. The formed vapor in said heater is separated from the aqueous urea solution in the liquid/gas after which said vapor is condensed in the condenser to form a carbamate solution. The released condensation heat is usually dissipated in cooling water. The formed carbamate solution in that recovery section operated at a lower pressure than the pressure in the synthesis section is preferably returned to the urea synthesis section operating at synthesis pressure. The recovery section is generally a single section or can be a plurality of recovery sections arranged in series.
In a urea stripping plant operating with carbon dioxide as a stripping gas, it is normally advantageous to introduce substantially all of the carbon dioxide into the synthesis loop via the stripper. In the event of the integrated production of urea and melamine, however, part of the carbon dioxide feed is determined by the recirculation of the off-gas of the melamine production. Since this off-gas contains both carbon dioxide and ammonia, it is less suitable to be used as a stripping medium, as using it would not result in a decrease of the partial pressure of only one if the components in the liquid phase.
With part of the carbon dioxide reactants thus being introduced elsewhere into the urea synthesis section, the carbon dioxide feed to the stripper will be necessarily reduced as compared to a urea plant operating on a stand-alone basis, i.e., without being integrated with the production of melamine. This results in a less efficient operation of the stripper. Whilst this could be offset by increasing the stripping temperature, the latter results in a higher overall energy consumption of the plant, typically in the form of a higher steam requirement (steam being used at the shell-side of the stripper to supply heat). Also, adding more heat to the stripper can only be done by increasing the stripper temperature. However this is limited because higher temperatures decrease the corrosion resistance of the materials from which such strippers are generally fabricated. Accordingly, increasing the stripper temperature increases corrosion, which may cause damage to said stripper, thereby reducing the lifetime thereof.
The invention seeks to provide integrated urea and ammonia production allowing stripping efficiency, steam consumption, or both, to be optimized.
This has not been adequately addressed in the art, despite a vast number of disclosures regarding various ways of integrating the production of urea and melamine. As examples reflecting the state of the art, reference is made to the following documents.
In WO98/08808 A1 a process for the integrated production of urea and melamine is illustrated in the block diagram as given in FIG. 2. In the known process, a gas stream originating from a high pressure process for the preparation of melamine is supplied directly to a high pressure section of a stripping process for the preparation of urea. A disadvantage of the known method is that stable operation of the resulting combined process for the preparation of melamine and urea is difficult: pressure fluctuations in one of the processes can easily affect the other process via the gas stream and thus result in unstable operation and process disorder. Another disadvantage is that the best operation of the known method is achieved if the melamine process has a higher pressure than said high pressure section of the urea process. As mentioned the energy consumption of the urea plant that processes the off-gas of an integrated melamine process increases in relation to a stand-alone urea plant. More specifically, as explained above, the energy consumption increases if the urea plant is a carbon dioxide or ammonia stripping plant type.
Another method is disclosed in U.S. Pat. No. 7,893,298 B2 and is illustrated in the block diagram as given in FIG. 4. In the known process, a gas stream originating from a high pressure process for the preparation of melamine is condensed in an aqueous ammonium carbamate stream that has been formed in a CO2 stripping process for the preparation of urea.
Yet another method is disclosed in WO 2008/052640 A1. In this known process the urea plant contains a medium pressure treatment section including a decomposer. A gas stream from melamine production is fed, together with vapor as formed in said decomposer and with the aqueous ammonium carbamate solution as formed in a downstream urea recovery section, to a condenser of the medium pressure treatment section. This results in a concentrated aqueous carbamate solution that is recycled to the high pressure urea synthesis section.
The invention seeks to reduce the steam consumption of a urea plant, after being integrated with a melamine plant. Alternatively, the invention seeks to keep the steam consumption for the urea production section in an integrated urea and melamine plant at least equal. All in all, the invention thus seeks to increase the economics and functionality in a facility for the integrated production of urea and melamine.